Hot Cracking -
The weld metal solidification starts from the fusion line towards the centre of the weld and in this process, concentrations of alloying elements and impurities arc pushed ahead of the inward growing crystals or dendrites.
The last stages of solidification, where dendrites interlock, arc critical because low freezing point (discontinuous) liquid films separating newly formed crystals produce a weak and brittle region. Cracking will easily result if transverse thermal strains are high. Solidification cracking is caused in the weld metal itself by tearing of the grain boundaries before complete solidification has taken place and while the metal is still in the plastic state.
Low-freezing point liquid films are either caused by impurities such as sulphur and phosphorus in steels or by alloying elements which create a wide freezing range such as Mg in Al/Mg alloys. It has been observed that those alloys which possess a long brittle range are sensitive to weld cracking, whilst those, having a short brittle range are not.
